Molecular Electronics

The primary contender for the post-silicon computation paradigm is molecular electronics, a nanoscale alternative to the CMOS transistor. Eventually, molecular switches will revolutionize computation by scaling into the third dimensionovercoming the planar deposition limitations of CMOS. Initially, these switches will substitute for the transistor bottleneck that results from a standard silicon process using standard external input/output interfaces.

For example, Nantero, a nanotech firm based in Woburn, Massachusetts, employs carbon nanotubes suspended above metal electrodes on silicon to create high-density nonvolatile memory chips (the weak Van der Waals bond can hold a deflected tube in place indefinitely with no power drain). Carbon nanotubes are small (approximately 10 atoms wide), 30 times as strong as steel at one-sixth the weight, and they perform the functions of wires, capacitors, and transistors with better speed, power, density, and cost. Cheap nonvolatile memory enables important advances, such as "instant-on" PCs.

Other companies, such as Hewlett-Packard and ZettaCore, are combining organic chemistry with a silicon substrate to create memory elements that self-assemble using chemical bonds that form along prepatterned regions of exposed silicon.

There are several reasons molecular electronics is the next paradigm for Moore's Law:

• Size: Molecular electronics has the potential to dramatically extend the miniaturization that has driven the density and speed advantages of the integrated circuit (IC) phase of Moore's Law. In 2002, using a scanning tunneling microscope (STM) to manipulate individual carbon monoxide molecules, IBM built a three-input sorter by arranging those molecules precisely on a copper surface. It is 260,000 times as small as the equivalent circuit built in the most modern chip plant. For a memorable sense of the difference in scale, consider a single drop of water. There are more molecules in a single drop of water than in all the transistors ever built. Think of the transistors in every memory chip and every processor ever built; there are about 100 times as many molecules in a drop of water. Certainly, water molecules are small, but an important part of the comparison depends on the 3-D volume of a drop. Every IC, in contrast, is a thin veneer of computation on a thick and inert substrate.

• Power: One of the reasons that transistors are not stacked into 3-D volumes today is that the silicon would melt. The inefficiency of the modern transistor is staggering. It is much less efficient at its task than the internal combustion engine. The brain provides an existing proof of what is possible; it is 100 million times as efficient in power and calculation as our best processors. Sure, it is slow (less than 1 kHz), but it is massively interconnected (with 100 trillion synapses between 60 billion neurons), and it is folded into a 3-D volume. Power per calculation will dominate clock speed as the metric of merit for the future of computation.

• Manufacturing cost: Many of the molecular electronics designs use simple spin coating or molecular self-assembly of organic compounds. The process complexity is embodied in the synthesized molecular structures, and so they can literally be splashed on to a prepared silicon wafer. The complexity is not in the deposition nor the manufacturing process nor the systems engineering. Much of the conceptual difference of nanotech products derives from a biological metaphor: Complexity builds from the bottom up and pivots about conformational changes, weak bonds, and surfaces. It is not engineered from the top down with precise manipulation and static placement.

• Low-temperature manufacturing: Biology does not tend to assemble complexity at 1,000 degrees in a high vacuum. It tends to work at room temperature or body temperature. In a manufacturing domain, this opens the possibility of using cheap plastic substrates instead of expensive silicon ingots.

• Elegance: In addition to these advantages, some of the molecular electronics approaches offer elegant solutions to nonvolatile and inherently digital storage. We go through unnatural acts with CMOS silicon to get an inherently analog and leaky medium to approximate a digital and nonvolatile abstraction that we depend on for our design methodology. Many of the molecular electronic approaches are inherently digital, and some are inherently nonvolatile.

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